The Cochrane Collaboration has a review on Homoeopathic Oscillococcinum for preventing and treating influenza ...
The conclusion - "Homoeopathic Oscillococcinum does not prevent influenza but might shorten the length of the illness Influenza (the flu) is a highly infectious respiratory disease caused by viruses. Other than treatments for complications (such as pneumonia) conventional medical treatment is bed rest. Homeopathy is a system based on 'curing like with like', often using highly diluted substances. Oscillococcinum is a homoeopathic preparation manufactured from wild duck heart and liver (common sources of influenza). It is claimed that Oscillococcinum (or similar homeopathic medicines) can be taken either regularly over the winter months to prevent influenza or as a treatment. Trials do not show that homoeopathic Oscillococcinum can prevent influenza. However, taking homoeopathic Oscillococcinum once you have influenza might shorten the illness, but more research is needed"